In wavelength modulation spectroscopy (WMS) the wavelength of a tunable light source, usually a continuously tunable laser such as a diode laser, is modulated by a small amount about an interaction feature of a sample to be measured, for example a molecular absorption line in a gas sample. The modulation frequency is f0. As the light interacts with the sample, for example propagates through a gas sample, wavelength dependent interaction such as absorption converts some of the wavelength modulation into an amplitude modulation of the light. Thus, the light will have an overtone spectrum generated by the interaction process, the harmonic content of the spectrum being depend on the interaction feature, for example the width and shape of the molecular absorption line in the gas and the etalons in the system. When the light impinges onto a detector, for example a photodiode, the detector output contains AC components at the modulation frequency f0 and its higher harmonics Nf0 (N=2, 3, 4, etc.). Demodulating the detector output at one of said higher harmonics Nf0 shifts the measurement from frequencies near DC, where the light source is noisy, into a higher frequency range, where the noise is lower, thus improving the measurement sensitivity.
From US-A-6040914 a wavelength modulation spectroscopy method and system are known in which a portion of the light of a tunable light source passes through a sample to a measuring detector, whereas another portion of the light directly impinges onto a monitor detector. The measuring detector output and the monitor detector output are fed to an autobalancing circuit, the output of which is demodulated at the double frequency Nf0.
US-B-635 1309 shows a further wavelength modulation spectroscopy method and system, in which a portion of the light is directed through the sample to the measuring detector and in which another portion of the light is directed through reference gas to a reference detector, the reference gas containing the gas species to be measured in the sample. There is however no monitor detector for directly detecting the light of the light source.
Another wavelength modulation spectroscopy method and system which are known from US-B-6356350 do not have a monitor detector either.
To realize a high measurement sensitivity requires the use of expensive highly linear diode laser sources. It would be a great advantage to be able to use inexpensive and widely available communication grade lasers instead, but the non-linearity of the diode laser generates harmonics interfering with the detected harmonics and induces a fluctuation of the baseline, which limits the sensitivity of the measurement.
The noise sources of a diode laser can be divided into deterministic noise and random noise. The most prominent random noise sources are shot noise and flicker (1/f) noise. The deterministic noise has its source of origin in the non-linearity of the laser I-P characteristics (I is the injection current and P is the output power) and the coupling between the laser and fiber and in other optical components. The sensitivity obtained in tunable diode laser spectroscopy is mainly limited by deterministic noise with the random noise being several orders of magnitude lower. The interference phenomena introduced by the deterministic noise are manifested as optical wavelength selective fading causing generation of Nf0 harmonics similar to the ones caused by the wavelength dependent interaction of the sample to be measured.